This invention relates to thermoplastic films for packaging; and more particularly, this invention relates to a multi-layer film having high moisture barrier characteristics.
Thermoplastic film, and in particular polyolefin materials, have been used for some time in connection with packaging of various articles including food products which require protection from the environment, an attractive appearance, and resistance to abuse caring the storage and distribution cycle. Suitable optical properties are also desirable in order to provide for inspection of the packaged product after packaging, in the distribution chain, and ultimately at point of sale. Optical properties such as high gloss, high clarity, and low haze characteristics contribute to an aesthetically attractive packaging material and packaged product to enhance the consumer appeal of the product.
Various polymeric materials have been used to provide lower moisture permeability in order to reduce the transmission of moisture through the packaging film and thereby extend the shelf life of products such as food, medical, electronic, and other items which are sensitive to moisture gain or loss. For some products, maintenance of a high moisture content is desirable, and the film ideally minimizes loss of moisture from the package to the environment. For other products, maintenance of a low moisture content is desirable, and the film ideally minimizes gain of moisture from the environment through the packaging material.
It is also often desirable to include in a packaging film a shrink feature, i.e, the propensity of the film upon exposure to heat to shrink or, if restrained, create shrink tension within the packaging film. This property is imparted to the film by orientation of the film during its manufacture. Typically, the manufactured film is heated to its orientation temperature and stretched in either a longitudinal (machine) or transverse direction (i.e. monoaxial orientation), or both directions (i.e. biaxial orientation), in varying degrees to impart a certain degree of shrinkability in the film upon subsequent heating. When biaxial orientation is done, it can be simultaneous or sequential; that is, the orientation can be done in each of the directions in turn, or else both the longitudinal and transverse orientation can be done at the same time. Any suitable technique, such as blown bubble or tenterframing, can be used to achieve orientation of the film. After being so stretched, the film is rapidly cooled to provide this latent shrinkability to the resulting film. One advantage of shrinkable film is the tight, smooth appearance of the wrapped product that results, providing an aesthetic package as well as protecting the packaged product from environmental abuse. Various food and non-food items may be and have been packaged in shrinkable films.
It is sometimes also desirable to orient a packaging film and thereafter heat set the film by bringing the film to a temperature near its orientation temperature. This produces a film with substantially less shrinkability, while retaining much of the advantages of orientation, including improved tensile strength, modulus and optical properties.
It is an object of the present invention to provide a thermoplastic multilayer film characterized by good moisture barrier properties.
It is a further object of the present invention to provide a thermoplastic multilayer film having an aesthetic appearance with good clarity, and other desirable optical properties.
It is another object of the present invention to provide a thin thermoplastic multilayer film having toughness and abrasion resistance.
It is still another object of the present invention to provide a thermoplastic multilayer film which may be totally coextruded, oriented, and have good moisture barrier and, in some cases, both moisture barrier and oxygen barrier properties.
Of interest are U.S. Pat. No. 4,921,749 (Bossaert et al); U.S. Pat. No. 5,085,943 (Crighten et al); U.S. Pat. No. 5,091,237 (Schloegel et al); U.S. Pat. No. 5,128,183 (Buzio); and U.S. Pat. No. 5,212,009 (Peiffer et al) disclosing the use of hydrocarbon resins.
The present invention relates to a thermoplastic multi-layer film comprising a core layer comprising a blend of propylene polymer or copolymer, or ethylene alpha olefin copolymer, and a hydrocarbon resin; and two outer layers comprising a propylene polymer or copolymer, ethylene alpha olefin copolymer, polybutene, or blends thereof.
In another aspect, the present invention relates to a thermoplastic multi-layer film comprising a core layer comprising a blend of propylene polymer or copolymer, and a hydrocarbon resin; two intermediate layers, on opposite surfaces of the core layer, comprising propylene polymer or copolymer, or a polymeric adhesive; and two outer layers comprising a propylene polymer or copolymer, ethylene alpha olefin copolymer, ionomer, polybutene, or blends thereof.
The present invention also relates to a thermoplastic multi-layer film comprising a core layer comprising an ethylene alpha olefin copolymer, ethylene propylene copolymer, rubber modified ethylene propylene copolymer, or ethyene propylene butane terpolymer; two intermediate layers, on opposite surfaces of the core layer, comprising a blend of propylene polymer or copolymer, and a hydrocarbon resin; and two outer layers comprising a propylene polymer or copolymer, ethylene alpha olefin copolymer, ionomer, polybutene, or blends thereof.
A film with oxygen barrier as well as moisture barrier properties comprises a core layer comprising an oxygen barrier material; two intermediate layers, on opposite surfaces of the core layer, comprising a polymeric adhesive; two outer layers comprising a blend of propylene polymer or copolymer, or polybutene, and a hydrocarbon resin; and a polymeric sealant layer adhered to at least one of the outer layers.
An alternative film with at least seven layers comprises a core layer comprising high density polyethylene; two intermediate layers, on opposite surfaces of the core layer, comprising a polymeric adhesive, ethylene vinyl acetate copolymer, or ethylene alpha olefin copolymer; two outer layers comprising a blend of propylene polymer or copolymer, and a hydrocarbon resin; and two outermost layers comprising propylene polymer or copolymer, polybutene, or blends thereof.
Another alternative is a film comprising a core layer comprising an oxygen barrier material; two intermediate layers, on opposite surfaces of the core layer, comprising polyamide; two tie layers, each disposed on a respective polyamide layer, comprising a polymeric adhesive, ethylene alpha olefin copolymer, or ethylene vinyl acetate copolymer; two moisture barrier layers, adhered to respective adhesive layers, comprising a blend of propylene polymer or copolymer, and a hydrocarbon resin; and two outermost layers comprising a propylene polymer, propylene copolymer, ethylene alpha olefin copolymer, or polybutene.
In another aspect of the invention, a method of making a thermoplastic multilayer film comprises the steps of coextruding an interior layer of a blend of propylene polymer or copolymer, and a hydrocarbon resin, and two outer layers comprising a propylene polymer or copolymer, ethylene alpha olefin copolymer, polybutene, or blends thereof; cooling the coextruded multilayer film; and collapsing the cooled film.
xe2x80x9cHydrocarbon resinxe2x80x9d (xe2x80x9cHCxe2x80x9d herein) and the like as used herein means resins made by the polymerization of monomers imposed of carbon and hydrogen only. Thermoplastic resins of low molecular weight made from relatively impure monomers derived from coal-tar fractions, petroleum distillates, etc. are also included. A discussion of HC resins can be found e.g. in U.S. Pat. No. 4,921,749 (Bossaert et al); U.S. Pat. No. 5,091,237 (Schloegel et al); and U.S. Pat. No. 5,128,183 (Buzio).
xe2x80x9cEthylene alpha olefin copolymerxe2x80x9d (EAO) is used here to include such materials as linear low density polyethylene (LLDPE); very low and ultra low density polyethylene (VLDPE and ULDPE); and metallocene catalyzed polymers such as those supplied by Exxon. Tafmer (tm) materials supplied by Mitsui are also included. These materials generally include copolymers of ethylene with one or more comonomers selected from C4 to C20 alphaolefins such as butene-1, hexene-1, octene-1, etc. in which the molecules of the copolymers comprise long chains with relatively few side chain branches or cross-linked structures. This molecular structure is to be contrasted with conventional low or medium density polyethylenes which are more highly branched than their respective counterparts. xe2x80x9cLLDPExe2x80x9d as defined here has a density in the range of from about 0.916 grams per cubic centimeter to about 0.940 grams per cubic centimeter. Generally, xe2x80x9cEAOxe2x80x9d as used here includes both homogeneous and heterogeneous polymers. xe2x80x9cIntermediate layerxe2x80x9d and the like is used herein to define a layer in a multi-layer film enclosed on both sides by other layers, i.e. disposed between other layers of the film.
The term xe2x80x9corientedxe2x80x9d and the like is used herein to define a polymeric material in which the molecules have been aligned in the longitudinal and/or transverse direction by a process such as a tenter frame or blown bubble process.
xe2x80x9cPropylene polymerxe2x80x9d and the like is used here to mean polymerized propylene in its homopolymer form, and xe2x80x9cpropylene copolymersxe2x80x9d means copolymers such as ethyene propylene copolymer, where generally small amounts of a comonomer such as ethylene are included in the copolymer. Terpolymers are also included.
xe2x80x9cPolymeric adhesivexe2x80x9d and the like here means polymeric materials, of any suitable composition, which can be used to create or enhance interlaminar bonds in multilayer thermoplastic films. Polyolefins are preferred, especially those which have been modified, e.g. by carboxylic acid or acid anhydride in a graft copolymer.
xe2x80x9cRubber modified as EPCxe2x80x9d and the like here means an EPC which has been modified by the inclusion of other moieties in the polymer structure. Such material may provide improved elasticity or other properties. An example is believed to be Himont KS-052 P, or those available from Rexene (El Paso).
xe2x80x9cEthylene propylene butene terpolymerxe2x80x9d and the like is used here to mean a terpolymer incorporating these three comonomers in various percentages. An example is KT-021 from Himont, or those available from Sumitomo.
xe2x80x9cCore layerxe2x80x9d as used herein means an intermediate layer of a multilayer film, and the central layer where the film has an odd number of layers.